More sophisticated IDSs look for common shellcode signatures.

They encrypt the shellcode by XORing values over the shellcode, using loader code to decrypt the shellcode, and then executing the decrypted shellcode

They convert the shellcode into Unicode, using loader to convert back to machine code then executing them

They reverse the working instructions into opposite order by masking the IDS signatures

They compress shellcode into normal instructions, uncompress the shellcode using loader code and then executing the shellcode